高效四次谐波转换技术及传输应用研究
文献类型：学位论文
| 作者 | 姜秀青 |
| 学位类别 | 博士 |
| 答辩日期 | 2016 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 林尊琪 |
| 关键词 | 非线性光学 谐波转换 四倍频 DKDP晶体 激光诱导损伤 |
| 其他题名 | Research of Efficient Fourth Harmonic Generation and Propagation Application |
| 中文摘要 | 在激光惯性约束聚变（ICF－Inertial Confinement Fusion）物理实验中，短波长激光可作为探针光束对等离子体的状态进行精密测量，利用Thomson散射原理对等离子体的时间和空间状态参数进行测量，由于Thomson散射的截面非常小(σth = 0. 665×10-24cm2 )为了避免探针光的吸收和光波色散的影响，探针光束波长必须足够小（以有效增加散射探测灵敏度），短波长激光能够更加精密的测量到等离子状态，记录的图像更为清晰。在ICF实验研究中，利用短波长激光直接驱动或者间接驱动，均可提高激光的吸收效率。 高功率激光系统中利用非线性光学中谐波转换过程可实现不同波长的输出，四次谐波的波长为263nm，相对于目前广泛使用的三次谐波（351nm）和二次谐波（527nm）具有更短的波长，由于临界相位匹配四次谐波转换对角度和环境要求较高，以及光学元件损伤问题突出，四次谐波在激光惯性约束聚变研究中作为驱动激光的应用发展受到限制，近几年晶体材料生长和非临界相位匹配技术的发展，再次引起人们对四倍频光的重视，使四倍频激光在未来ICF系统中作为打靶激光成为可能。深入研究四次谐波的高效输出方法和应用对四倍频激光作为激光惯性约束聚变研究中驱动激光具有重要意义。 本论文分析了高功率激光的谐波转换特点，数值模拟了谐波转换过程，利用非临界相位匹配技术实现了四次谐波的输出；针对短波长激光传输中问题进行了初步研究，包括材料的双光子吸收，损伤和晶体的横向受激拉曼散射等，主要包括以下几方面工作： 1. 模拟了高功率激光驱动器中谐波转换过程，着重分析了高功率激光驱动器中KDP晶体倍频混频过程、相关特点及应用。针对利用部分掺氘DKDP晶体实现非临界相位匹配四次谐波转换，首次建立了非临界相位匹配条件下四次谐波转换DKDP晶体掺氘量与温度的关系模型，并经过实验验证。根据关系模型可以推出一定掺氘浓度下晶体的非临界匹配温度，亦可根据晶体的非临界匹配温度反推出晶体的掺氘浓度。 2. 实验研究了掺氘浓度为70%和65%的DKDP晶体非临界相位匹配四次谐波转换。晶体分别在温度为17.7℃和29.4℃下实现了非临界相位匹配，与掺氘量和温度的关系模型吻合良好；实验测得这两种浓度四倍频晶体的接收角宽分别为53mrad和55mrad，远大于KDP晶体临界相位匹配的接收角宽（3mrad）；实验中倍频激光为光强0.05GW/cm2的高斯脉冲，对于65%的掺氘DKDP晶体，得到25%的倍频到四倍频转换效率。 3. 独立设计了一套口径为50mm×50mm晶体夹持精密温控装置。利用水循环系统均匀改变晶体的温度，温度控制在±0.1℃以内，晶体温度分布均匀面积可达40mm×40mm。利用该温控装置夹持厚度10mm的65%掺氘DKDP晶体，实现了神光II高功率激光装置非临界相位匹配下四倍频光的输出。基频光为光强为0.94GW/cm2的梯形脉冲，基频光到四倍频光的转换效率为43%，倍频到四倍频转换效率达70%，可实现高效四倍频转换。四倍频激光应用在Thomson散射诊断实验，用于诊断nc/4处等离子体温度。 4. 模拟分析了晶体的厚度，温度，泵浦光的带宽和汇聚光路对非临界相位匹配下四倍频转换效率的影响，提出了非临界相位匹配下四倍频激光靶场终端概念设计。针对光强为2~2.5GW/cm2的倍频泵浦光，采用厚度为5mm的70%掺氘DKDP晶体，放置在F数大于30的汇聚光路中，晶体温度应控制在非临界相位匹配温度（17.7℃）±0.2℃以内，基频泵浦光带宽控制在0.01nm内，可实现转换效率达80%的四倍频激光输出。 5. 针对四倍频激光传输问题，搭建了一套稳定的四倍频激光输出平台，利用该平台进行了263nm激光诱导损伤测试实验，研究不同紫外光学材料对四倍频激光传输的影响。通过测量材料透过率获得Heraeus-suprasil 312 熔石英样品和CaF2样品的双光子吸收系数；利用损伤测试平台，实验测得Corning-7980熔石英样品、Heraeus-suprasil 312 熔石英样品和CaF2样品在脉宽为5ns、波长为263nm的高斯脉冲激光辐照下的损伤阈值和损伤增长阈值；针对不同材料的激光损伤特性，研究了熔石英和CaF2材料的损伤形貌特征。 |
| 英文摘要 | In the physical experiment of Inertial Confinement Fusion (ICF), short wavelength laser can be used as a probe beam to measure the states of the plasma precisely. Thomson scattering is a key diagnostic condition due to its ability to provide accurate temporal-and spatial-resolved information. Because of the tiny section of Thomsen scattering(σth = 0. 665× 10-24cm2 ), the probe beam wavelength is required to be as short as possible to avoid the influence of the absorption and distortion. The short wavelength laser can be used in Thomsen scattering diagnostic experiment to provide more precisely information of the plasma and more clearly images. In the ICF experiment, the short wavelength laser can be used as the diving laser to improve the laser absorption efficiency, no matter it is the direct drive or indirect drive method. In the high power laser system, a variety of wavelength lasers can be obtained by the means of harmonic conversion using nonlinear crystals. The wavelength of fourth harmonic conversion is 263nm, which is much shorter comparing with the widely used third harmonic laser (351nm) and second harmonic laser (527nm). The application of fourth harmonic laser has been limited in ICF experiment due to its high requirement about the critical phase matching angle and environment stability, and the low damage threshold of optical elements under fourth harmonics laser. With the development of the crystal material growth and noncritical phase matching technology, the fourth harmonic conversion laser draws people more attention, which makes it possible to be used as the driving laser in the future ICF system. The deep research on the high efficiency fourth harmonic conversion method and application as diving laser in ICF experiments has great significance. This paper analyzed the characteristics of the harmonic conversion in high power laser system, numerical simulation analysis is carried out about the harmonic conversion process; noncritically phase-matched fourth-harmonic generation of Nd:glass lasers with partially deuterated DKDP crystal has been confirmed; a preliminary study is undertaken on the propagation application of short wavelength laser, including two photon absorption, laser damage and the transverse stimulated Raman scattering. The main contents and innovative results are as following: 1. The second, third and fourth harmonic conversion process in high power laser system are simulated. The process, characteristics and the application of the harmonic conversion using KDP crystals are analyzed. The relationship model between the deuterium content of DKDP crystal and temperature of noncritically phase matched fourth harmonic generation is established, and it is proved by the experiment. The noncritically phase matching temperature can be inferred by this model while knowing the particular deuterium content of DKDP crystal, and it works reversely. 2. The fourth harmonic conversion under the noncritical phase matching condition is realized by the precisely control of the DKDP crystal temperature. The fourth harmonic conversion under the noncritical phase matching condition is realized at 17.7 oC and 29.4oC using DKDP crystal of 70% and 65% Deuterium content separately. The NCPM FWHM angular acceptance of these two DKDP crystals are ~53 mrad and 55mrad, which are much wider than the angular acceptance for critically phase-matched KDP crystal (3.3mrad). The conversion efficiency from second harmonic laser to fourth harmonic laser is 25%, while the second harmonic laser is a Gauss pulse laser with intensity of 0.05GW/cm2. 3. A precisely temperature control facility for 50mm×50mm crystal is designed. The temperature of crystal can be controlled within ±0.1 oC by the water cycle system, and the area of uniformed temperature distribution can reach 40mm×40mm. The noncritically phase-matched fourth harmonic generation is demonstrated in SG II high power laser facility using a 10mm long 65% deuterated DKDP crystal, which is place in the temperature control facility. The conversion efficiency from fundamental frequency laser to fourth harmonic laser is 43%, while the fundamental frequency laser is a trapezoidal pulse laser with intensity of 0.9 GW/cm2, and the conversion efficiency from second harmonic laser to fourth harmonic laser is about 71%. The fourth harmonic laser is used as probe beam in Thomson scattering diagnostic experiments for diagnosing plasma temperature at nc / 4. 4. The optical properties of fourth harmonic laser and beam splitter in laser focal path have been analyzed, an optical assembly to generate fourth harmonic laser based on NCPM is designed. While the intensity of second harmonic laser is 2~2.5 GW/cm2, a 5mm long 70% detuterated DKDP crystal is placed in the laser focal path, the F values are larger than 30, the temperature of crystal is controlled within 0.2oC around nocritically phase-matched temperature (17.7 oC), and the bandwidth of fundamental frequency laser is controlled within 0.01nm, the conversion efficiency for second harmonic laser to fourth harmonic laser can up to 85%. 5. Due to the research of propagation of fourth harmonic laser, a stable platform of fourth harmonic laser was established, , the damage experiment of 263nm laser was carried out on the platform, and the affection of different UV optical elements on the transmission of fourth harmonic laser was analyzed. By testing the transmission rate, the two photon absorption coefficients of Heraeus-suprasil 312 fused silica and CaF2 were measured. The damage threshold and the initial damage threshold of growth of Heraeus-suprasil 312 fused silica and CaF2 at 263nm, 5ns pulse-width laser were measured. The damage morphologies of fused silica and CaF2 were analyzed. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15973]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 姜秀青. 高效四次谐波转换技术及传输应用研究[D]. 中国科学院上海光学精密机械研究所. 2016. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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